The invention relates to a Laser Feedback Control System. The control system utilizes algorithms of modern digital controls and utilizes a laser controller Integrated Circuit. Use of edge emitter lasers for high performance long haul and metropolitan networks will continue in high performance applications including wavelength division multiplexing. VCSEL arrays have been used increasingly in fiber optics telecommunication due to their low cost. Widespread use will place increasing demands on performance of all elements of the electrical to optical interface. Increasing speed of computers will place increased demands on the performance of transmitters using VCSELs. All network applications are a fast growing market. The market is demanding low cost and short development time with an increased level of reliability and intelligence in the transmission systems.Prior art has attempted various methods to control lasers. Levinson, in U.S. Pat. No. 5,019,769, dated May 28, 1991, described a semiconductor laser diode controller and laser diode biasing control method. Although a programmed microcontroller is disclosed, a limitation to Levinson is the teaching is to only “ . . . accurately controlling the process of turning on and selecting the operating point of the laser diode.” More specifically, Levinson is directed “ . . . preventing light from the laser diode's from accidentally damaging user's eyes.” Levinson does not appear to disclose an integrated circuit solution. The micro controller is used in a hardware adjustment mechanism rather than for servo computations. Levinson teaches away from the present invention. On the other hand, in the present invention, algorithms handle the entire set of controls in firmware and do not rely on analog closed loop controls. This feature allows for advanced controls, which precisely stabilize the laser, can accommodate adaptive controls and can be leveraged from one laser transmitter design to another.King, et al. In U.S. Pat. No. 5,812,572, dated Sep. 22, 1998, discloses intelligent fiberoptic transmitters and methods of operating and manufacturing the same. Intelligent fiberoptic/laser diode transmitter/controller modules and methods of operating and manufacturing the same are disclosed.“During calibration procedures for the modules, a laser diode is characterized over a defined operating temperature range.” . . . “During operation, an embedded microcontroller together with analog to digital converters, digital to analog converters and other associated circuitry, dynamically control the operational parameters (e.g. modulation and bias current) based on the current operating conditions (temperature, power supply).” A limitation of King, et al is the limited scope of the disclosure. King, et al appears to teach away from an integrated circuit solution. More specifically, a micro controller is used in a hardware adjustment mechanism rather than for servo computations. In addition, King, et al discloses characterizing of a laser diode. On the other hand, the present invention uses algorithms to handle the entire set of controls in firmware and does not rely on analog closed loop controls. This feature allows for advanced controls, which precisely stabilize the laser, can accommodate adaptive controls and can be leveraged from one laser transmitter design to another. The present invention also discloses an operating system, advanced servo control methods, and adaptive, reconfigurable controls in an integrated control system.
Still another patent in prior art is Sanchez, in U.S. Pat. No. 6,494,370, dated Dec. 17, 2002. Sanchez is the inventor of the present invention as well. However, U.S. Pat. No. 6,494,370, is an electro-optic system controller and method of operation disclosing a method for calibrating a laser module system. This disclosure is more limited in scope than the present invention because it focuses on some hardware aspects of controls.
Other prior art is discussed below.
Some control systems as shown in FIG. 1, utilize analog controllers. Adjustment of laser power with changes in temperature is done by a temperature sensitive device, which adds compensation current to the laser drive. This approach is made with fixed controls that need to be hardwired for each application. Because of that reason this implementation reduces the possibilities for modifications and the possibilities for leveraging from one product generation to another.
Other methods are utilized for controlling the more complex applications of wavelength division multiplexing WDM have utilized some elements of closed loop control but generally only for stabilizing the wavelength of the laser with a wavelength locker.
Other approaches consist of open loop drivers with costly characterization of the laser/VCSEL array.
An example of a more elaborate control system is shown in FIG. 2. The system accommodates for an initial characterization of the laser at all temperatures. In this control system, a photodiode-monitoring laser output power is used as feedback. The photodiode characteristic is digitally adjusted at the factory with a D/A converter and a microprocessor. This adjustment is in lieu of a mechanical potentiometer and it is used for an initial adjustment of the photodiode and to compensate for deficiencies of the photodiode with temperature. It is significant that in FIG. 2, the control loop is still an analog control loop. The difference between the loop in FIG. 2 and a digital control loop is that in a digital control loop, the magnitude of the laser drive signal is determined in a digital process. For example, the photodiode signal is first digitized, the processor then utilizes calibration and adjustment algorithms to determine what the drive signal magnitude should be, and finally, a D/A converter is used to control the driver so it produces the correct magnitude of the drive current. In contrast, in a digital control system, the signals are immediately digitized, all of the signal processing for a servo system is carried out in the controller firmware and then the excitation/control of the laser is done with the D/A converters controlling the driver. This allows for complex signal processing to be done in the firmware.
The analog circuits utilize complex analog topologies, which are difficult to leverage, support and calibrate. The analog parts also require a significant effort in maintaining component quality in production.
Other deficiencies of analog control systems are in the way the control algorithms are implemented. Adjustments are made for power control but utilize very basic methods of control because a significant set of the decisions and calculations are done with analog components. For example, the laser power control is done with a system that makes adjustments based on deviations from a set point with a resulting in on-off adjustments rather than a servo system.
Prior solutions do not effectively leverage solutions from one product to another because they are based on fixed hardwired design implementations of a control system.
Prior solutions rely on a multiplicity of integrated circuits and components that are chosen every time the laser system is designed. This situation causes a larger set of packages to be utilized increasing costs and decreasing reliability.
Another issue of the known prior art methods is that complex issues need to be decided very early in the projects. With rigid hardwired implementations (rather than programmable which can be implemented late in the project) the options are reduced thus increasing risk.